1) Field of the Invention
The present invention relates to power generating equipment, particularly, to power generating equipment in which electric power is generated with the aid of solid oxide fuel cells.
2) Related Art Statement
Recently, fuel cells have been noted as a power generating source. The fuel cell is capable of directly converting chemical energy possessed by fuel to electric energy. Since the fuel cell is free from the limitation of Carnot's cycle, the fuel cell essentially has a high energy conversion efficiency. Further, various fuels such as naphtha, natural gas, methanol, coal reformed gas and heavy oil may be used. Additionally, these fuels may be used with a low pollutant level. Moreover, the power generating efficiency of the fuel cells is not influenced by the scale of the equipment. Therefore, power generating equipment using solid oxide fuel cells is an extremely promising technique.
Particularly, since the solid oxide fuel cell (hereinafter abbreviated as SOFC) operates at a high temperature of 1000.degree. C. or more, the activity of electrodes thereof is very high and the use of a noble metal catalyst such as expensive platinum is not required. In addition, since the SOFC has a low polarization and a relatively high output voltage, the energy conversion efficiency is considerably higher than that of the other fuel cells. Furthermore, since the SOFC is constructed with solid materials, it is stable in structure and has a long life.
FIG. 1 is a cross-sectional view showing an example of a conventional power generating equipment using hollow-cylindrical type SOFCs.
In FIG. 1, a round bottomed hollow-cylindrical SOFC element 5 comprises a round bottomed cylindrical porous support tube 6, an air electrode 7 formed on an outer periphery thereof, and a solid electrolyte 8 and a fuel electrode 9 arranged around the outer periphery of the air electrode 7 in this order. Generally, a plurality of such SOFC elements 5 are connected in series and parallel to constitute a collected cell, and the collected cell is provided at a given position in a power generating chamber 13. Only one SOFC element is drawn in FIG. 1 for ease of explanation. At the lower side of the power generating chamber 13, a fuel gas chamber 14 is provided; and the fuel gas chamber 14 is separated from the power generating chamber 13 by a bottom side division wall 11. Additionally, At the lower side of the fuel gas chamber 14, a heat insulating wall 12 is provided. On the other hand, at the upper side of the power generating chamber 13, a gas exhaust chamber 3 is arranged, which is separated from the power generating chamber 13 by an upper side division wall 4. In the upper side division wall 4, a hole 4a is provided, through which an opening side end portion of the SOFC element 5 is inserted. At the upper side of the gas exhaust chamber 3, a heat insulating wall 1 is arranged having hole 1a, through which an oxidizing gas supply tube 2 is inserted so as to be held by the hole 1a. Then, a top end opening 2a of the oxidizing gas supply tube 2 is positioned in an internal space 10 of the SOFC element 5.
When operating the hollow-cylindrical SOFC, an oxidizing gas is supplied into the oxidizing gas supply tube 2 as shown by an arrow A, then the flow of the oxidizing gas supplied from the tube 2 is converted at the bottom portion of the element 5 and the gas is fed through the internal space 10 to be exhausted into the gas exhaust chamber 3 as shown by arrows B. On the other hand, when a fuel gas is supplied into a fuel gas chamber 14 through a fuel gas supply hole 12a formed in the heat insulating wall 12 as shown by an arrow C, pressure in the fuel gas chamber 14 becomes high; then the fuel gas is supplied into the power generating chamber 13 through the fuel gas supply holes 11a formed in the division wall 11, as shown by arrows D. The fuel gas is then fed in an upper direction along the surface of the fuel electrode 9 of the SOFC element 5. Oxygen ions diffused in the solid electrolyte 8 are reacted with the fuel gas on the surface of the fuel electrode 9. As a result, an electric current is fed between the air electrode 7 and the fuel electrode 9. The used fuel gas is exhausted into the gas exhaust chamber 3 through the space 4a formed between the upper side division wall 4 and an opening end portion of the SOFC element 5, as shown by arrows E. The power generating equipment having no sealing as shown in FIG. 1 is preferred to be used, because the SOFC element 5 is operated under a high temperature about 1000.degree. C.
In order to put such power generating equipment in practical use, it is necessary to decrease the manufacturing cost therefore and to increase the power density of the equipment. Therefore, i t is necessary to make the length of the SOFC element 5 long to increase the power generating output per one SOFC element.
However, in the power generating equipment having its constitution as shown in FIG. 1, there is a drawback that a temperature gradient is generated in the power generating chamber 13 due to the concentration gradient of the fuel gas flow in the chamber 13. That is to say, in the vicinity of the through holes 11a, through which the fuel gas is supplied into the chamber 13, the content of fuel is still large, so that a large amount of fuel is consumed by an electrochemical reaction there and hence the temperature of the atmosphere in the vicinity of the through holes 11a increases. Due to the temperature increase, the electrochemical reaction of the oxygen ions and the fuel on the fuel electrode 9 becomes more intense.
On the other hand, at a distant portion from the fuel supply holes 11a, the concentration of the fuel gas becomes low, and then the fuel amount consumed electrochemically is also decreased. Therefore, the temperature at the upper portion of the fuel electrode 9 and the electrochemical reaction are not elevated and remain relatively low. Additionally, a large amount of CO.sub.2, steam, etc. is contained in the fuel gas, whose concentration has been reduced, and the CO.sub.2 or steam adheres on the surface of the fuel electrode 9 to obstruct the reaction. Thus, the electrochemical reaction becomes increasingly inactive along the upper side of the chamber 13.
Therefore, there is a large temperature gradient which is generated between the upperstream side and the downstream side of the fuel gas flow in the chamber 13. When such power generating equipment is operated for a PG,7 long time, the temperature gradient not only causes cracks on the SOFC element 5 but also decreases the power generating efficiency thereof. This tendency becomes more considerable as each of cylindrical SOFC elements 5 is lengthened.